(1) Field of the Invention
This invention relates to an optical disk used for data recording and reproduction, especially to a multi-layered optical disk having multiple recording layers.
(2) Description of the Prior Art
In recent years, this type of optical disks have been developed actively due to the large memory capacity and high access speed. An optical disk shown in FIG. 1 has been proposed in order to further increase the memory capacity.
This optical disk 12 comprises three recording layers 8a through 8c formed of a photochromic material such as spyropyrene, the layers being interposed between a pair of bases 13. The recording layers 8a through 8c have sensitivity peaks in wavelengths xcex1 through xcex3 (FIG. 2), respectively while allowing lights having the other wavelengths to transmit therethrough.
Data recording and reproduction is done in the following way. A light is emitted from a light source 9, such as a laser, which varies wavelengths, and focused into an extra fine light beam by a focusing optical system 10, thereafter the light is illuminated on the disk 12. The light is transmitted through the recording layers 8a, 8b and 8c and is detected by a light detector 11 provided on the other side from the light source 9.
Data recording will be described in more detail. If the light emitted from the light source 9 and illuminated on the disk 12 has a wavelength xcex2, it is transmitted through the recording layers 8a and 8c but is absorbed into the recording layer 8b, whereby a data is recorded in the layer 8b. 
For data reproduction, only the data recorded in the layer 8b can be retrieved by illuminating a light of xcex2.
As apparent from the above, memory capacity is increased by providing more recording layers.
However, providing more recording layers enlarges the total thickness of the recording layers. In order to record and reproduce data in such a thick disk only by use of wavelength difference without detecting exact positions of the layers, the light beam should have quite a large diameter, which prevents high density recording.
Also, the large light beam diameter causes crosstalks between neighboring tracks.
Accordingly, the present invention has an object of offering a multi-layered optical disk which detects an exact position of each recording layer for minimizing the diameter of the light beam and thus remarkably enhancing the recording density.
This invention has another object of offering a multi-layered optical disk which prevents crosstalks between neighboring tracks and layers.
The above objects are fulfilled by a multi-layered optical disk comprising a plurality of recording layers accumulated in the thickness direction wherein a light beam is focused on one of tracks of one of the layers thereby to record and reproduce data, the optical disk being characterized in that recording layers each have an identification section storing an address of the recording layer which the identification section belongs to.
The identification section may store an address of the track which the identification section belongs to.
The tracks of two of the layers neighboring in the thickness direction may be shifted against each other in the radial direction by half of a track pitch.
The optical disk may have two recording layers.
The tracks each may comprise a plurality of sectors.
The sectors each may have an identification section, which stores addresses of the recording layer, the track and the sector which the identification section belongs to.
The identification sections may be shifted against one another in the tracking direction.
The above objects are also fulfilled by a multi-layered optical disk comprising a plurality of recording layers each having a plurality of tracks, wherein the layers are accumulated in the way that the tracks are aligned in the thickness direction; the optical disk being characterized in that at least one of recording layers has a first identification section storing an address of the tracks which are aligned in the thickness direction and one of which has the first identification section; and that the recording layers each have a second identification section storing an address of the recording layer which the second identification section belongs to.
The first identification section may have long enough a pit to allow a recorded data to be reproduced if the light beam is focused on either one of the recording layers while the second identification section has short enough a pit to allow the recorded data to be reproduced if the light beam is focused on the layer specified.
The tracks each may comprise a plurality of sectors, each of which has its address stored in the first identification sections
The above objects are also fulfilled by a multi-layered optical disk comprising a plurality of recording layers each having a plurality of tracks, wherein the layers are accumulated in the way that the tracks are aligned in the thickness direction; the optical disk being characterized in that at least one of recording layers has a first identification section storing an address of the tracks which are aligned in the thickness direction and one of which has the first identification section; and that the recording layers each have a second identification section storing an address of the recording layer which the second identification section belongs to, the second identification sections being shifted against one another in the tracking direction.
In the above construction, since each layer of the optical disk has its own address stored in the identification section thereof, the exact position of the desired recording layer is easily found. As a result, the diameter of the light beam can be minimized, realizing high density recording.
Moreover, when the identification sections of the layers neighboring in the thickness direction are provided so that the light beam may not be focused on two or more of the sections simultaneously, crosstalks between neighboring identification sections can be substantially prohibited. Therefore, the desired identification section, namely, the desired recording layer, can be accurately detected.
In conclusion, the above construction provides high precision, high density recording on multiple layers of an optical disk.